The invention relates to an FM-receiver comprising an aerial input having connected thereto, in succession, an RF input stage, an IF portion, an FM-detector and a stereo decoder and also comprising a control signal generating circuit and a mono-stereo control circuit connected thereto, the control signal generating circuit comprising a multipath reception detector and being connected to the IF portion via a level detector, the multipath reception detector comprising an amplitude detector connected to the level detector via a bandpass filter.
Such an FM-receiver is disclosed in German patent application No. 2 929 647, which has been laid open to public inspection.
The prior art FM-receiver comprises a mono-stereo switching circuit which at a low field strength and/or a high degree of multipath reception, effects a mono reproduction and at a high field strength and a low degree of multipath reception, effects a stereo reproduction. The audible noise produced by poor signal reception is thereby reduced at the cost, of a spatial reproduction. With the prior art FM-receiver, the multipath reception is measured by amplitude detection of the output signal of the level detector within the passband of said bandpass filter, the level detector being connected to the IF-portion. This passband may either be wide-banded (extending from some KHz to approximately 100 KHz) or narrow-banded around 19 KHz or 47 KHz. The field strength is measured by an integration of the output signal of the said level detector. The control signal for the mono-stereo switching circuit is obtained by subtracting the output signals of the multipath reception detector and the field strength detector from each other, after a mutual amplitude match.
Pulse-shaped, artificial interferences (so-called man-made noise) caused, for example, by electric ignitions of internal combustion engines, may however produce noise components which pass said bandpass filter and are included in the measurement of the multipath reception. The control signal for the mono-stereo switching circuit of the prior art FM receiver is consequently partly determined by these noise pulses. Even in good receiving conditions, that is to say where a low degree of multipath reception and a high field strength occur, these noise pulses may cause frequent and/or prolonged mono reproduction. This indeed somewhat reduces the annoying effect of the noise pulses on the reproduction, but it also causes an audible loss of spatial reproduction which, particularly in the case of rapidly repeating mono change-over actions may result in a spatially instable sound impression of continuously springing sound sources.